1. Field of the Invention
The present invention relates to a method for recognizing an abnormal image, and more particularly to a method for recognizing an abnormal image which is generated by LBB or shading.
2. Description of the Related Art
Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes a light source, a linear array of photoelectric sensing elements (generally a CCD sensor or a CMOS sensor, or a CIS sensor), an analog amplifier, an analog to digital converter (ADC), a controller and a random access memory (RAM).
The CCD sensor includes a large number (e.g., 2000) of photoelectric sensing elements arranged in a linear array. Each photoelectric sensing element will capture light representing a single pixel of the image. The array will capture a line of pixels. By moving the CCD sensor across a document, the entire document can be scanned one line at a time.
The conversion into digital signals of light reflected from or transmitted through the document takes place in essentially three steps. First, each photoelectric sensing element will convert the light which it receives into an electric charge. The magnitude of the charge will depend on the intensity of the light and the exposure time. Second, the charges from each of the photoelectric sensing elements are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized by the analog to digital converter for digital image processing and storage in the RAM.
As shown in FIG. 1, a conventional image scanner is shown. A light beam is emitted from a light source 102 and reflected by a reflector 104. The light beam then exposes a document sheet 112 and is reflected by the document sheet 112. The reflected light beam then is reflected sequentially by mirrors 106a and 106b. The image of the document sheet 112 carried by the light beam is transmitted to charge-coupled devices 110 through a lens 108. Under perfect circumstance, the image of a document sheet should not present dark lines or bands in a bright field as well as bright lines or bands in a dark field. As shown in FIG. 2, a dark band 202 and several dark lines 206 show up a bright field 202. The width of the dark band 204 is larger than a pixel and the dark band 204 is called LBB. The dark line 206 having a width equal to a pixel is called shading. LBB is usually induced from obstacles in the transmitting path of the light beam between the charge-coupled devices 110 and the light source 102. The obstacles could be, for example, some dust on the mirrors 106a and 106b or the lens 108. Shading is usually caused by bad pixels (CCD) in the charge-coupled devices 110. It is therefore that the reasons separately causing LBB and shading are extremely different since one relates to malfunction of devices and the other dose not. Accordingly, it is important for an operator or a manufacturer of an image scanner to recognize and tell LBB from shading or vice versa since individual maintenance level needed is contrary. It is desirable to provide a method for recognizing abnormal image which is generated by LBB or shading so as to render an operator or a manufacturer of an image scanner easier to restore normal functions of the image scanner.